An optical parametric oscillator (OPO) is a light source emitting radiation with properties comparable to that of a laser. OPOs are nonlinear devices that split high frequency pump photons into two lower frequency photons, namely signal and idler photons. The frequencies of the signal and idler photons are not independent from each other, but may be tuned in frequency.
OPOs need an optical resonator, but in contrast to lasers, OPOs are based on direct frequency conversion in a nonlinear optical element rather than from stimulated emission. OPOs exhibit a power threshold for an input light source (pump), below which there is negligible output power in the signal and idler bands.